For heavy vehicles, e.g. bus, truck and the like, tubeless tires are widely used.
In such a heavy duty tire, the engaging force between the tire beads and the bead seats of the rim must be large to maintain airtightness. This is particularly important when the tire load and inner pressure are very large. However, as shown in FIG. 10, so called bead toe lifting (f), a phenomena that the bead toe (a) is lifted from the bead seat (b) though the tire is mounted on its regular rim and inflated to a normal pressure and loaded by a normal load, was often observed.
It was found that such a toe lifting reduces not only the airtightness but also bead durability. Further, it was found that the bead durability decreases in proportion to the increase in the amount (L) of the bead toe lifting (f) as shown in FIG. 9.
In the tires in which bead toe lifting occurred, the larger the service pressure, the greater the bead toe lifting. Accordingly, the deformation of the bead portion was larger, and damage occurred earlier.
In the bead portion, the tensile stress of the carcass produced by the tire inner pressure is generally radially outward and axially outward. Therefore, in the ground contacting patch of the tire, the bead core is forced toward the rim flange by the axial component of the tensile stress, which component is in direct proportion to the amount of tire deflection. On the other hand, the radial component of the tensile stress of the carcass increases as the measuring position is circumferentially apart or away from the ground contacting patch, and the axially inside of the bead core is forced radially outwardly. The direction of the resultant force to which the bead core is subjected is changed during running. Therefore, the axially outward bead rubber portion is compressed axially by the axially outward movement of the bead core and radially by the radially inward movement of the carcass turned up portion. On the other hand, the axially inward bead rubber portion is pulled by the carcass main portion. As a result, under a heavy load and high pressure condition, the bead rubber is subjected a repeated large stress, and the rubber is permanently deformed. Accordingly, the airtightness between the tire beads and bead seats of the rim are decreased. Further, such permanently deformed rubber is liable to be cracked especially at points in the radially outward bead rubber portion near the carcass ply turnup edge and the radially outer edge of the rim flange. The bead durability is greatly decreased by such a crack.